1. Field of the Invention
The invention relates to methods for extending the spectral bandwidth of an excitation signal of a speech signal, methods for reconstructing noisy parts of a speech signal recorded in a noisy environment, and methods for enhancing the quality of a speech signal.
2. Related Art
Speech is the most natural and convenient way of human communication. This is one reason for the great success of the telephone system since its invention in the 19th century. Today, subscribers are not always satisfied with the quality of the service provided by the telephone system, especially when compared to other audio sources, such as radio, compact disk or DVD. The degradation of speech quality using analog telephone systems is often caused by the introduction of band limiting filters within amplifiers employed to keep a certain signal level in long local loops. These filters typically have a passband from approximately 300 Hz up to 3400 Hz and are applied to reduce crosstalk between different channels. However, the application of such bandpass filters considerably attenuates different frequency parts of the human speech ranging from about 0 Hz up to 6000 Hz.
Great efforts have been made to increase the quality of telephone speech signals in recent years. One possibility to increase the quality of a telephone speech signal is to increase the bandwidth after transmission by means of bandwidth extension. The basic idea of these enhancements is to establish the speech signal components above 3400 Hz and below 300 Hz and to complement the signal in the idle frequency bands with this estimate. In this case the telephone networks can remain untouched.
Additionally, mobile communication systems such as cellular phones have been developed in recent years and are employed in different environments. By way of example, cellular phones are often employed in vehicles or in other environments where a strong background noise exists. In vehicle applications, a hands-free speaking system is often employed to avoid diverting the attention of the driver from the traffic while using the cellular phone.
Additionally, speech recognition systems have been developed that are also often employed inside vehicles. These systems are able to control different functions of the vehicle. In these systems, the speech recognition system needs to recognize the commands and other audio inputs of the driver, the recorded signal comprising speech components and noise components. The same is true for hands-free systems, in which the recorded speech signal from the driver also includes noise components from the background noise inside the vehicles.
In both systems, when a telephone call is received via a telecommunication system having a limited bandwidth or when speech is recorded in a noisy environment, there exists the problem that certain frequency ranges are either not present in the transmitted signal or are heavily distorted. On the other hand, a speech signal having an extended frequency range could be better understood. Accordingly, the speech quality in the above-mentioned scenarios (e.g., in very high noise conditions) where traditional methods such as noise suppression systems do not work properly needs to be improved. Therefore, a need exists to provide a method for restoring a signal for which a certain frequency part is missing.